Motor Bearing Damage and Variable Frequency Drives ... Bearing... · Motor Bearing Damage and Variable Frequency Drives: - Diagnosing the ... •Electrical arc damage similar to electric

Motor Bearing Damage and Variable Frequency Drives:- Diagnosing the Causes,- Implementing a Cure, and- Avoiding the Pitfalls

Tim Albers, Director of Product Mgt, NIDEC Motor Corporation [email protected]

Tim Jasina, Sr. Application Engineer / Engr. Team Leader [email protected]

Ken Fonstad, HVAC Sales / Application Support Principal Engineer [email protected]

mailto:[email protected]



Anatomy of an AC induction motor

AC current in the stator windings produces a rotating magnetic field.

This generates a current in the rotor bars, which makes its own magnetic field.

The interaction of the two magnetic fields produces the motor’s torque.

2

The frequency of the AC in the stator controls the speed of the motor.

Operation of an AC induction motorV

olt

age

AC Line-to-Line Voltage

L1-L2 L1-L3 L2-L3

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How does a variable frequency drive (VFD) work?

VFD

4

How does a variable frequency drive (VFD) work?

VFD

Rectifier DC Bus Inverter

5

The voltage from the PWM VFD can change from minimum to maximum in less than 1 µs (0.001 ms)

Fast-rising pulses

The voltage of the AC power line changes gradually from minimum to maximum in about 8 ms

How is a VFD different from the AC power line?V

olt

age

AC Line-to-Ground Voltage

L1 L2 L3

PWM Voltage Pulses

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Fast-rising pulses

• Fast-rising voltage pulses can readily induce voltage across the air gap between the motor’s stator and its rotor

• This can drive stray currents in the motor

How is a VFD different from the AC power line?

7

Air gap

Circulating currents

• Asymmetries in the motor can result in a circulating current between the motor’s frame and the motor’s shaft

• This seldom happens in motors < 50 HP; more common in 480 V and higher voltage AC motors

Cause of current through a motor’s bearings

8

Common mode voltage

• When powered from a balanced center-grounded wye transformer, the net voltage with respect to ground is always 0 V (no common mode voltage)

• Common mode voltage can induce voltage on the motor’s shaft

What causes bearing currents in smaller motors?V

olt

age

AC Line-to-Ground Voltage

L1 L2 L3

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Common mode voltage

• When powered from a VFD, only two voltages (+ & -) make the three-phase voltage to the motor

• The 3-phase voltage applied to the motor isn't balanced with respect to ground

What causes bearing currents in smaller motors?

10

+

−

Common mode voltage

• If the stator has a net positive voltage,it will induce a positive voltage onto the shaft

• If the voltage on the shaft is high enough, current will arc through the grease in the bearings

What causes bearing currents in smaller motors?

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+

+

−

• Noise from the bearings

That’s a bit late

• Motor vibration

That’s a bit late and difficult

• Measure the current

That’s hard to do

• Measure the shaft voltage

There’s an idea

How can you detect the onset of bearing damage?

12

Measuring shaft voltage

• Use a brush to contact the motor’s shaft

• Home-made: fine steel wool brush

• Commercial: carbon fiber brush


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• Use a magnetic stand to hold the brush in place


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• Use a digital storage oscilloscope to capture the motor shaft voltage

• Use the trigger function to capture the highest shaft voltages

• Save the waveforms in computer files


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Interpreting the voltage waveforms


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• The shaft voltage smoothly increases and then quickly drops due to a discharge

28.8 V peak

1 µs/div

Interpreting the voltage waveforms


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• The shaft voltage oscillates, but no sharp drop is detected

• No shaft current was present

12.8 V peak

500 ns/div

Remotely detecting bearing discharges

• When a discharge occurs, it generates a pulse of electromagnetic noise, like radio static

• A hand-held radio frequency detector can detect the “static” and count the discharges


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Non-contact detection of bearing discharges

• Advantages

• Little set-up is required

• Counts the discharges over time

• Concerns

• Proper positioning of the meter is required for accurate readings

• Practice and experience are required


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Not all bearing damage is caused by bearing current

• There are also mechanical causes for bearing damage

• Before implementing a solution, it is important to understand the cause of the problem

• Study the damage to the bearings

• Ideally, the grease in the bearing should not be cleaned away before the bearing is submitted for evaluation

Identifying the cause of bearing damage

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Images of damaged bearings

• Classic bearing fluting, likely caused by shaft voltage discharges


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• Damage occurs when current passes through bearing

• Oil film between balls and raceways acts as an insulator

• Film is only microns thick at most

• Shaft voltage can exceed ‘dielectric strength’ (when insulating property fails) of film

• Arcing occurs – melting a minute area of surfaces

• Melted metal enters grease or is rolled over onto raceways

• Initially forms a dull grey ‘frosted’ or matte appearance

• Fluting can develop with continued arcing


0.003mm

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Bearing electrical arc damage process

• A frosted ball path appears

• Potentially electrical arc damage

• Potentially lubrication contamination

• Extreme magnification required to verify

• Scanning electron microscope

• Confocal microscope


0.007mm

Initial electrical arc damage

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Advanced electrical arc damage – Fluting

• Overlapping arcs

• Fluting’s repetitive appearance related to dynamic system of arcing and rolling motion

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• Electrical arc damage similar to electric discharge machining (EDM) process

• Bearing surface material removed


0.0012mm

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• Likely caused by shaft voltage discharges, made worse by a large radial load

• With light radial loading, the insulating grease layer is uniform and thick

• A large radial load reduces the thickness of lubricant in the load zone


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Possible solutions for this problem

• Belt drive application:

• Reduce the belt tension

• Use larger diameter sheaves on the motor and the load

• When the load is shaft-mounted:

• Mount the load as close to the motor as possible

• Reduce the weight of the load

• Use external bearing(s) to support the load

• Increase the minimum speed


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Why increase the minimum speed?

• Test on three motors with radial loads


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0 V

10 V

20 V

30 V

40 V

50 V

33% 50% 67% 83% 100%Be

arin

g In

sula

tio

n B

reak

do

wn

Vo

ltag

e

Motor Speed

Effect of Motor Speed onBreakown Voltage

Motor 1

Motor 2

Motor 3

Not all bearing damage is caused by bearing current

• The motor made a “clicking” noise when it was rotated slowly

• A fluting pattern is too dense to make such a sound

• The shaft voltage and hand-helddetector showed no discharges


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2.84 V

5 µs/div

Study of the bearings

• The racesof the bearing showed regularly spaced marks

• The spacing of the markswas the same as the spacing of the balls in the cage of the bearing


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Diagnosis: False Brinelling

• The motor was driving a shaft-mounted fan wheel

• It appears that the problem was caused by vibration during shipping


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Other mechanical damage: Brinelling

• Brinelling is generally caused by a sharp impact on the bearing

• This causes an indent in the bearing’s race

• It often indicates damage during the assembly of the equipment


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• Not all bearing damage is the result of electrical discharges through the bearings

• Mechanical damage

• Lubrication problems

• Too little

• Too much

• Contamination

• Overloading

• It is important to know the problem before developing a solution


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Summary

A variety of solutions are available

1. Proper wiring

2. VFD adjustments

3. System maintenance

4. High frequency toroids

5. Blocking the discharge

6. Re-routing the discharge

Avoiding bearing discharge currents

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1. Proper wiring

• The VFD sends pulses with high frequency content to the motor

• It is important to provide a low impedance return path for the high frequencies to protect the …

• … motor bearings

• … the building from radio frequency noise


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1. Wiring: only a safety ground

• The VFD sends pulses with high frequency content to the motor

• It is important to provide a low impedance return path for the high frequencies to protect the …

• … motor bearings

• … the building from radio frequency noise


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100%

Radio frequency noise in the facility

1. Wiring: only a safety ground

Lab test

• 60 ft cable


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24 V

100%

Discharge!

1. Wiring: traditional ground, motor to VFD

• Normally sized ground wire

• No conductive conduit


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70%

30%

1. Wiring: large ground and conductive sheath

• 1-3 large ground wires with large strand count

• Ground wires are close to the motor wires; this reduces magnetic fields to minimize impedance

• Grounded, conductive shield around all of the wires


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5* - 10%

* With VFD’s RFI/EMC filter engaged

90 – 95*%

1. Wiring: large ground and conductive sheath

Lab test

• 60 ft cable


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5* - 10%

0.92 VNo discharge

90 – 95*%

1. Proper wiring

• Advantages

• No maintenance required

• Can have a major impact

• Concerns

• Requires attention to details

• VFD cable can be expensive, but it is not absolutely necessary; carefully installed conduit can be used


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2. VFD adjustments

• Enter the motor nameplate data

• Activate energy optimization in the VFD

• Both minimize the motor current

• Reduce the switching frequency

• Cutting the pulse rate reduces the number of possible discharges

• Increase the minimum frequency

• Helps center a radially-loaded shaft

• A “sleep” function could stop the motor automatically

Reducing bearing discharge currents

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2. VFD adjustments

• Advantages

• Easy to implement

• Concerns

• Mostly only extends the life of the bearings; doesn’t eliminate the problem


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• Check belt tension

• Check shaft alignment

• Check balancing of the load

• Check bearing lubrication

• Check environmental concerns


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• Advantages

• Should be done anyway

• Concerns

• May not be the problem


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• A high frequency toroidal coreconcentrates the magnetic field of high frequency currents passing through it, impeding their flow

• It can be used in two ways:

• To combat common mode voltage and current

• To combat the high frequency components of pulses to the motor


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• Common mode filter

• Built into the VFD


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There are other designs of common mode filters


• Common mode filter

• Added between the VFD and the motor


VFD

The ground wire does not pass though the toroid

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• High frequency filters, for circulating currents

• Added between the VFD and the motor


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The ground wire does not have a toroid around it

VFD


• Advantages

• No maintenance required

• Can be retrofit in most situations

• Concerns

• Requires proper wiring between the VFD and the motor


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5. Blocking the discharge: insulating sleeve

• Doesn’t need to block a particularly high voltage

• There are a number of ways to block the flow of current through a motor’s bearings

• Add an insulating sleeve between the bearing and the motor


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5. Blocking the discharge: insulating sleeve

• Advantages

• Blocks current flow through the bearing

• Concerns

• For retrofits, requires disassembling and machining of the motor

• Conductive dirt across the sleeve

• For common mode shaft voltage, both bearings must be insulated

• May transfer bearing damage to the connected load unless an insulated coupling is used


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5. Blocking the discharge: insulated bearings

• Insulated bearings are of two general types:

• With rolling elements made of a non-conductive material, such as a ceramic

• Using an inner and/or outer ring with an electrically insulated mounting surface

• In most case, this does not impact the mounting dimensions of the bearing


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5. Blocking the discharge: insulated bearings

• Advantages

• Blocks current flow through the bearing

• Can be a drop-in replacement

• Concerns

• For common mode shaft voltage, both bearings must be insulated

• May transfer bearing damage to the connected load unless an insulated coupling is used

• When the rolling element is non-metallic, the ratings of the bearing may be impacted


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• By electrically “grounding” the motor’s shaft to the motor’s frame, shaft voltage can be dissipated around the bearingas long as the low impedance conductive path is maintained

• A number of methods are available


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• Rotary contact

• Spring-loaded carbon brush

• Conductive fiber brush


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• Advantages

• Can generally be retrofitted in the field

• For common mode voltage, only one may be needed

• Concerns

• For circulating current, using only one only makes the problem worse

• Often, insulating the other bearing is recommended


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• Concerns, continued

• For large motors, multiple devices may be needed

• Proper mounting and shaft preparation are essential

• Environmental conditions may impact its operation

• May require periodic inspection and/or maintenance


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This band formed under the brushes.

Review

• Background on motors, VFDs and bearing currents

• How to detect bearing current

• How to diagnose causes of bearing damage

• How to avoid damage due to bearing current

Questions?

VFDs and Motor Bearing Damage

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Documents

Motor Bearing Damage and Variable Frequency Drives ... Bearing... · Motor Bearing Damage and Variable Frequency Drives: - Diagnosing the ... •Electrical arc damage similar to electric